There has been growing interest in asymmetric ultrafiltration (UF) membranes derived from block copolymers since the first report by Peinemann et al. (Asymmetric superstructure formed in a block copolymer via phase separation. Nat. Mater. 2007, 6, 992-996.) resulting in isoporous membranes that combine a highly ordered surface structure with high permselectivity. The most widely studied polymer system fabricated to date with this block copolymer self-assembly and non-solvent induced phase separation (SNIPS) process is the diblock copolymer, poly(styrene-b-(4-vinyl)pyridine) (PS-b-P4VP, SV). The phase inversion technique was successfully applied to other diblock copolymer systems and triblock terpolymer systems.
Employing a second component in the casting dope solution, various membrane properties were tailored. For example, the morphology of the top surface layer was tuned by the addition of small organic molecules and metal ions, which form metal-polymer complexes. With the introduction of an additive that chemically interacts/swells one block of the block copolymer, pore sizes were tailored. For example, an organic additive, glycerol, was added to the ISV system in order to tailor the pore size moving the range of filtration from UF to nanofiltration. Pore sizes were also tailored in ISV terpolymer derived membranes by blending a homopolymer or in SV copolymer derived membranes by blending of other SV block copolymers with varied molar mass and block volume fractions. Finally, organic-inorganic hybrid membranes have been reported from the SNIPS process by mixing in sol- or other inorganic nanoparticles into the block copolymer dough, while carbon materials with asymmetric structure were derived from adding in resols and subsequent heat processing at elevated temperatures.
In order to design smarter membrane systems with tunable pore surface chemistry, it is desirable to use block copolymer architectures enabling attachment of foreign functional components, e.g. in a facile post-membrane-fabrication step. A critical challenge is the lack of effective incorporation methods. Several attempts have been made to achieve double-functionality through a binding layer, such as polydopamine, which enables other functional materials to adhere to membranes. This extrinsic binding layer, however, suffers from easy exfoliation during membrane usage. Also, the formation of a binding layer and its functionalization requires multiple post-processing steps. A co-assembly method to incorporate an inorganic component into the polymeric membranes was previously described, but this method only works when additives are compatible with organic solvents used for casting. Membranes with pores lined by acrylic acid moieties for which they anticipated further functionalization capabilities was previously described, again suggesting multiple post-membrane-fabrication steps. However, the use of acrylic acid as coupling sites was not demonstrated. Another challenge in this field is to expand on the diversity of chemistries present on the surface of the membrane pores. For example, it would be desirable to have membranes pores that simultaneously provide a stimulus response (e.g. pH dependent swelling and deswelling), binding sites for post-processing surface modifications, and hydrophilic groups to reduce fouling. To date most SNIPS derived membranes only provide a single pore surface functionality.